A typical fish is torpedo-shaped and streamlined. Fish have a muscular tail with a caudal fin that propels them forward and a pair of pectoral fins, placed just behind the gills, used for steering and balance. Most fish also have one to three dorsal fins on the back, an anal fin just below the tail and a pair of pelvic fins on the ventral surface. Species such as salmon and catfish also have an adipose fin, which is just fatty tissue without support, behind the dorsal fin.

Fish are covered with a slimy secretion that decreases friction with the water, and nearly all species have scales to keep the body waterproof. Fish scales are formed from bone and there are many different types. Ganoid scales are a rhomboid shape, heavy and plate-like, and are covered with an enamel-like substance called ganoine. Primitive fish such as sturgeon and gars have ganoid scales. Sharks have placoid scales or dermal denticles. These also have an enamel-like coating and have hard, spiny projections, giving the skin a rough finish. It is thought that vertebrate teeth evolved from placoid scales. Cycloid scales, present in bony fish such as carp, are flat and overlapping with smooth, rounded edges. Ctenoid scales have comb-like edges and also overlap.

There is a brilliant variety of colours and patterns among fish. Some are cryptically coloured as a camouflage and are able to change shade to match their environment. As in some bird species, the male is often brightly coloured and the female is drab in contrast. The male’s colouration may be used in courtship displays. Pelagic fish tend to have white bellies, silvery flanks and dark blue backs. This makes it difficult for prey and predators to spot them because it breaks up their outline when seen from a lateral perspective. When viewed from above, the darker shade blends in with the sea, and when viewed from below, the pale colouring merges with the light from the sky.

Fish have a network of sensory receptors located along the head and side which detect pressure changes in the water. This is called the lateral line system (see lateral line).

Gills are a fish’s external respiratory organs. Water is taken in via the mouth and then through gill slits and over the gills. The gill surface is well-supplied with blood vessels, over which oxygen and carbon dioxide diffuse. A few species of fish also have air-breathing lungs in addition to gills (see gills).

Fish are ‘cold-blooded’(poikilothermic), which means that their internal temperature is about the same as their external environment. This is in contrast to ‘warm-blooded’ (homeothermic) animals, such as mammals, who can regulate their internal temperature. This means that fish are often unable to survive in environments that warm-blooded animals can.